Disapearing walls

I have just registered an account on here and I need help with this problem, recently I have been playing around with hammer and sometimes randomly a wall or floor disappears for no reason, in hammer its there but in game its not, but sometimes its also missing in hammer at the same time I don't know what the problem is so could I please get some help

sorry but what is the cordon tool? I'm just using the run map to make a bsp, I made a box and hollowed it so yes and no?

Click to expand...

Look for one of these to be on, make sure that they are both off.

Signatures are an outdated concept that hold no bearing to forum conversations, but are given to people anyways to make them feel special and unique when in reality, they are special and unique, no matter if they have a signature or not.

ok so it happened again, I made a door in hammer and when I run the map its not there, I used the interlopers error checker, I fixed some leaks and the only thing left is this weird error:

can't load skybox file skybox/sky_tf2_04 to build the default cubemap!Description:
You have entered a skybox texture which doesn't seem to be valid. Make sure you don't specify the last three letter (_UP, _DN, etc) of any skybox texture you use. You may also need to fix your vmt files if they are broken. It may also be caused by compiling a non-HDR skybox texture.

but anyways I don't think that conflicts with my door disappearing, I also have an vmf file here for you

It's going to be a week, I'm on vacation. It's probably a very simple fix, I would suggest checking out some basics tutorials from TopHattWaffle on YouTube or any number of other written tutorials for "first map" type Hammer ventures on the Internet. Check out the VDC as well.